The proposed project will investigate the nature and significance of vowel misarticulations in the speech of children with moderate to severe phonological disorders. Vowel production has been relatively unstudied in children's phonological development and disorders. However, recent reports of phonologically disordered children exhibiting vowel misarticulations (without diagnosed motor or sensory deficits) have highlighted the importance of studying vowels (and their interaction with other components of the phonological system). The goals of the present study are to determine the relative incidence of vowel misarticulations among normally developing and phonologically disordered children, to provide detailed descriptions of vowel productions from several perspectives (linguistic, acoustic, and articulatory), to determine the relationship between vowel misarticulations, consonant misarticulations, and other subject characteristics, and to investigate the clinical significance of vowel misarticulations. The project will consist of three phases. In Phase I, data will be collected from 150 phonologically disordered children (from 3;6 to 6;5) and 150 normally developing children (from 1;6 to 6;5), using a protocol designed to assess all American English consonants and vowels in a variety of phonetic contexts in single word productions and continuous speech. These data will be phonetically transcribed and analyzed for consonant and vowel inventories, production accuracy and consistency, and error types. Results will be used to determine the frequency distribution and common patterns of vowel misarticulations in each group, and to investigate proposed relationships between consonants and vowels. In Phase II, 20 phonologically disordered children (identified in Phase I as exhibiting moderate to severe vowel misarticulations) will be selected for further analysis of their vowel productions in controlled stimuli using perceptual (transcription), acoustic (formant frequency, duration) and articulatory (linguapalatal contact) measures. Three control groups will also be tested, including (a) phonologically disordered children without vowel misarticulations matched for consonant production accuracy, (b) normally developing children matched for age, and (c) younger normally developing children matched for consonant production accuracy. Additional testing (e.g., vowel perception, oral structure/function, oral sensory perception, stimulability) will be conducted to determine what characteristics, if any, differentiate children with vowel errors from children without vowel errors. Phase III will directly address the clinical (diagnostic and prognostic) importance of vowel errors. First, subjects from Phase II will be followed longitudinally to determine the amount and type of change in vowel production without direct intervention. Second, experienced listeners will be asked to estimate perceived degree of severity of Phase II subjects. Finally, experimental studies using carefully controlled synthetic stimuli will examine the relative effect of vowel errors and consonant errors on intelligibility.